Radiation imaging apparatus and imaging method using radiation

ABSTRACT

There is provided a panoramic imaging apparatus which serves as a radiation imaging apparatus. The panoramic imaging apparatus includes an X-ray tube radiating an X-ray as a radiation, a detector outputting digital-quantity frame data corresponding to an incident X-ray, and moving means moving the pair of X-ray tube and the detector relatively to an object. The apparatus further includes means for acquiring the frame data from the detector during movement of the X-ray tube and the detector, and means for optimally focusing a portion being imaged of the object using the acquired data and producing a three-dimensional optimally focused image in which the real size and shape of the portion being imaged are reflected.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/386,838 filed on Jan. 24, 2012 which is a 371 U.S. National Stage ofInternational Application No. PCT/JP2010/062842, filed Jul. 29, 2010,which claims priority to Japanese Patent Application No. JP 2009-178415,filed Jul. 30, 2009. The disclosures of the above applications areentirely incorporated by reference herein.

BACKGROUND

1. Field of the Invention

The present invention relates to a radiation imaging apparatus andimaging method using the radiation, and in particular, to the radiationimaging apparatus and imaging method using the radiation in which and bywhich radiation data is obtained by radiation-scanning an object inplural directions, the obtained radiation data are processed with atornosynthesis method to reconstruct topographic data, the reconstructedtomographic data being used to identify three-dimensional positions ofinternal structures of the object.

2. Background Art

In recent years, tomographic imaging using a tornosynthesis techniquehas been used actively. The theory of this tornosynthesis technique hasbeen known long before (for example, refer to patent reference 1), andrecently, tomographic imaging that enjoys ease of image reconstructionperformed using the tornosynthesis technique has been proposed (forexample, refer to patent references 2 and 3). Especially, many suchcases can be found in dental and mammographic fields (for example, referto patent references 4, 5 and 6).

In the dental field, tornosynthesis technique is usually put intopractical use as a panoramic imaging apparatus that acquires panoramicimages in which a curved tooth row is usually expanded into atwo-dimensional plane. This panoramic imaging apparatus is usuallyprovided with a mechanism that rotates a pair of an X-ray tube and anX-ray detector around the oral cavity of an object being imaged. TheX-ray detector has pixels mapped in a rectangle of a portrait-orientedwidth. The mechanism rotates the pair of the X-ray tube and the X-raydetector with a rotation center thereof intricately so that the rotationcenter traces a predetermined orbit which is previously set along atooth row. The predetermined orbit is set to focus on a 3D referentialtomographic plane previously set along a tooth row which can be regardedas a tooth row having a standard shape and size. During the rotation, anX-ray beam is radiated from the X-ray tube at given intervals and theX-ray is transmitted through the object to be received by the X-raydetector, and digital frame data is detected from the detector. In thisway, the frame data focusing on the 3D referential tomographic plane isacquired at the given intervals. These frame data are subjected toreconstruction using the tornosynthesis technique so as to provide apanoramic image of the 3D referential tomographic plane.

However, the foregoing conventional panoramic imaging apparatuses do nottake it account the facts that there are differences between the toothrow of each object and the 3D referential tomographic plane andpositioning tooth rows involves difficult operations. As might beexpected, there are individual differences in the shapes and sizes ofthe tooth rows of respective objects. The sizes of objects' jaws differdepending on the individuals, making it difficult to correctly positionthe tooth rows. This often causes defocused panoramic images to bereconstructed, which may fail to meet a demand for fine interpretationof the images. In such cases, if it is desired to finely examine casesincluding cavities and alveolar pyorrhea, it is needed to performintraoral imaging or dental CT imaging, separately from the panoramicimaging. Re-performance of the panoramic imaging and X-ray imaging usinganother modality will raise the amount of X-rays to which the object isexposed.

In order to try to overcome such difficulties, there is provided anapparatus provided by patent reference 7. In the panoramic imagingapparatus shown in this publication, a phantom is used to previouslymeasure gains (i.e., distance information for mutual addition of framedata) and positions in each of depth directions of a tooth row.Additionally, acquired frame data are used to produce a focus-optimizedimage of the 3D referential tomographic plane using the tornosynthesistechnique. In the focus-optimized image, a ROI is set to specify apartial region, and a focus-optimized image at a selected position inthe front-back direction (i.e., each of front-back directions of thetooth row, which connect the X-ray tube and the X-ray detector at eachof the radiation positions) of the partial region are reconstructedusing already acquired frame data and a gain necessary among the gainswhich have been measured. Hence, the data acquisition is performed onetime with the 3D referential tomographic plane focused, and then, afocus-optimized image of any partial region can be reconstructed bymaking use of the already acquired frame data.

PRIOR ART REFERENCE Patent Reference

-   [Patent Reference 1] JP-A-S57-203430-   [Patent Reference 2] JP-A-H6-88790-   [Patent Reference 3] JP-A-H10-295680-   [Patent Reference 4] JP-A-H4-144548-   [Patent Reference 5] JP-A-2008-110098-   [Patent Reference 6] US2006/0203959 A1-   [Patent Reference 7] JP-A-2007-136162

However, the panoramic imaging apparatus provided by the patentreference 7 does not consider the fact that teeth being imaged arecurved or warped in their longitudinal directions. It is usual that eachtooth of a tooth row is not at the same longitudinal position. It isfrequent that the teeth are curved inwards in the oral cavity asadvancing to the base thereof. It is thus difficult to focus on all theareas of each tooth on one section. With consideration of this regard,it is necessary to focus on all the longitudinal areas of the teeth toraise depiction performance thereof. In other words, though theforegoing panoramic imaging apparatus can focus on any partial regionlocated at an arbitrary position in the front-back direction, it isdifficult to obtain one panoramic image which is focused through theentire areas of a tooth row. Even when best-focused partial images areconnected to represent an entire panoramic image, there are causedirregularities between connected edges of the partial images, thusspoiling the connection.

The foregoing difficulty is boosted by the fact that enlargement factorsin the longitudinal and lateral directions of an image (i.e., thelongitudinal and width directions of the tooth row) differ depending onchanges in the rotation center position during the scanning. Theenlargement factor is defined as a ratio between the actual size of atooth and the size of an enlarged image of the tooth, of which shadow isprojected to the X-ray incident surface of the X-ray detector. Since theX-ray tube has an X-ray source which is small enough which can beregarded as a point source, the X-rays are radiated from the point X-raysource. However, in reconstructing a tooth row in a 3D tomographic planebased on the tornosynthesis technique, the lateral enlargement of areconstructed image is the same at any positions on the image, but thelongitudinal enlargement thereof differs at the positions. This causes apanoramic image to be reconstructed longer than the actual size of atooth row in the lateral direction. In addition, the enlargement, thatis, the degree by which an image is longer longitudinally, differsdepending on where a tooth is located, i.e. in the anterior teeth orboth side molar teeth (i.e., back teeth), causing the longitudinalshapes thereof depending on the tooth is an anterior or molar teeth,thus causing distortion among the teeth in a panoramic image. Stillmore, when a tooth row is not entirely or partially along a given 3Dreferential tomographic plane, the distortion among respective portionsof the tooth row will be enhanced more severely due to differences inthe longitudinal and lateral directions.

In the conventional panoramic imaging apparatus in whichdigital-quantity frame data are acquired to reconstruct a panoramicimage thereon, post processing is often carried out to remedy theforegoing difficulty. In this post processing, a reconstructed image ismultiplied by coefficients which allow the sizes of the teeth to beshortened such that the ratio between the longitudinal and lateralenlargements becomes the same, at least, at the center of the anteriorteeth area. Even in such a remedy, the height of the molar teeth isdepicted smaller than the actual size thereof in the panoramic image.That is, there remains distortion among the individual teeth due to thefact that the enlargement factor differs positionally.

In this way, the difficulty resulting from differences of theenlargement factor has not been overcome, and optimally focusing on theentire region of an object in a panoramic image has not been realized.It is thus frequently difficult to interpret and diagnose teeth and/orthe gum depicted in the conventional panoramic image. Particularly it isdifficult to reliably measure lengths and distances in such images.Hence, for instance, implant placement is confronted with a difficultyof positioning implanted portions with less accuracy.

As a conventional countermeasure, a marker to indicate a referenceposition is attached to a desired position in the oral cavity beforeimaging, so that the reference position is given in an image. Withreference to the reference position, the image is corrected to maintainaccuracy, so that the foregoing difficulty is compensated as much aspossible. However, in this measure, steps for imaging and diagnosisbecome complex. An operational burden on operators is heavier. Thus, dueto such reasons, the marker cannot be used easily for preventivepractice such a screening test. It is therefore highly needed to providepanoramic images which can be used widely from the preventive measuresuch as screening to complicated treatment such as implant placement.

In addition, a three-dimensional panoramic image would be useful fordiagnosing the whole structure of a tooth row in its back-and-forthdirection. However, images which meet such a need and overcome thefreeing various difficulties have not been provided yet.

In consideration of the foregoing, it is an object of the presentinvention to provide a radiation imaging technique which is able tooptimally focus on the entire region of an object being imaged in athree-dimensional panoramic image in a state where the actual state(position and shape) of the object is three-dimensionally depicted withhigher accuracy and distortion due to changes in the enlargement factoris removed or reduced well.

SUMMARY

In order to achieve the object, the present invention provides aradiation imaging apparatus, a data processing apparatus, an imagingmethod using the radiation, and a computer program.

Of these, a radiation imaging apparatus, characterized in that theapparatus includes a radiation emitting source that emits a radiation; aradiation detector that output, frame by frame, digital-quantitytwo-dimensional data corresponding to incidence of the radiationthereto; moving means for moving a pair of the radiation emitting sourceand the radiation detector, the radiation detector, or an object beingimaged relatively to a remaining one among the radiation emittingsource, the radiation detector, and the object; data acquiring means foracquiring, frame by frame, the data outputted from the radiationdetector while the pair of the radiation emitting source and theradiation detector, the radiation detector, or the object is movedrelatively to the remaining one by the moving means; and image producingmeans for producing a three-dimensional optimally focused image based onthe data acquired by the data acquiring means, a portion to be imaged ofthe object in the three-dimensional optically focused image beingoptimally focused and reflecting an actual size and shape of the portionthereon.

Further, the data processing apparatus processing data outputted from asystem comprising: a radiation emitting source that emits a radiation; aradiation detector that output, frame by frame, digital-quantitytwo-dimensional data corresponding to incidence of the radiationthereto; moving means for moving a pair of the radiation emitting sourceand the radiation detector, the radiation detector, or an object beingimaged relatively to a remaining one among the radiation emittingsource, the radiation detector, and the object; and data acquiring meansfor acquiring, frame by frame, the data outputted from the radiationdetector while the pair of the radiation emitting source and theradiation detector, the radiation detector, or the object is movedrelatively to the remaining one by the moving means, characterized inthat the data processing apparatus comprises: data storing means forreceiving and storing therein the data; and image producing means forproducing a three-dimensional optimally focused image based on the datastored in the data storing means, a portion to be imaged of the objectbeing imaged to be optimally focused and to reflect an actual size andshape of the portion thereon.

In addition, the method of imaging using a radiation, characterized inthat the method comprises steps of: acquiring data, frame by frame,outputted from a radiation detector while a pair of the radiationemitting source and the radiation detector, the radiation detector, oran object being imaged is relatively moved relatively to a remaining oneamong the radiation emitting source, the radiation detector, and theobject, the radiation detector outputting, frame by frame,digital-quantity two-dimensional data corresponding to incidence of theradiation thereto; and producing a three-dimensional optimally focusedimage based on the data acquired by the acquiring step, a portion to beimaged of the object in the three-dimensional optically focused imagebeing optimally focused and reflecting an actual size and shape of theportion therein.

The computer-readable program stored in a memory and readable from thememory by a computer and produced to enable the computer to process dataoutputted from a system comprising: a radiation emitting source thatemits a radiation; a radiation detector that output, frame by frame,digital-quantity two-dimensional data corresponding to incidence of theradiation thereto; moving means for moving a pair of the radiationemitting source and the radiation detector, the radiation detector, oran object being imaged relatively to a remaining one among the radiationemitting source, the radiation detector, and the object; and dataacquiring means for acquiring, frame by frame, the data outputted fromthe radiation detector while the pair of the radiation emitting sourceand the radiation detector, the radiation detector, or the object ismoved relatively to the remaining one by the moving means, characterizedin that the program enables the computer to have functions realized bysteps of: reconstructing, as a referential-plane image, a projectionimage produced by projecting a desired referential tomographic plane ofa portion being imaged of the object to a detection surface of theradiation detector based on the data acquired; setting a plurality oftomographic planes along the 3D referential tomographic plane in adirection opposed to the 3D referential tomographic plane; calculatingpixel values of each of the plurality of tomographic planes based onpixel values of the referential tomographic plane; identifying optimallyfocused sampling points of the portion being imaged based on image dataof both the 3D referential tomographic plane and the plurality oftomographic planes to which the pixel values are provided; providing theidentified sampling points with pixel values based on the pixel valuesat the sampling points corresponding to the panoramic image and beingpresent in a viewing direction from the X-ray tube to the detector viathe respective sampling points; deciding the portion being imaged, byrecognizing the 3D referential tomographic plane provided at thesampling points to which the pixel values are provided and acharacteristic pattern shown by pixel values of the plurality oftomographic planes; removing noise from the sampling points of thedecided portion being imaged; and producing a three-dimensionaloptimally focused image including the portion being imaged with anactual size and shape of the portion reflected thereon, by connectingthe sampling points of the portion from which the noise is removed.

Effects of the Invention

According to the radiation imaging apparatus, the data processingapparatus, the imaging method using the radiation, and the computerprogram, acquired data are used to provide a three-dimensional optimallyfocused image in which a portion being imaged of an object is optimallyfocused and imaged to have an actual size and shape reflected thereon.That is, with the actual state (position and size) of the portiondepicted three-dimensionally with accuracy, the entire image isoptimally focused and is removed or reduced from being distorted due todifferences of the enlargement factor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view outlining the whole configuration of anX-ray panoramic imaging apparatus employed as a radiation imagingapparatus according to one embodiment of the present invention;

FIG. 2 is a view explaining the tooth row of an object being examined bythe panoramic imaging apparatus according to the embodiment, a 3Dreferential tomographic plane which is set at the tooth row, and atrajectory of a rotation center on which a pair of an X-ray tube and adetector rotates;

FIG. 3 is a perspective view explaining a geometry of the X-ray tube,the 3D referential tomographic plane, and the detector in the panoramicimaging apparatus;

FIG. 4 is a block diagram outlining the electrical and electronicconfiguration of the panoramic imaging apparatus;

FIG. 5 is a flowchart showing an outline of processing for imagingperformed cooperatively by a controller and an image processor providedin the panoramic imaging apparatus;

FIG. 6 is a view explaining a positional relationship among the X-raytube, the 3D referential tomographic plane, the rotation center, and thedetector;

FIG. 7 is a graph explaining a relationship between frame data andmapping positions of the frame data to produce a panoramic image;

FIG. 8 is a view pictorially showing an example of a reference panoramicimage;

FIG. 9 is a view pictorially showing an example of the referencepanoramic image on which a ROI is set;

FIG. 10 is a flowchart outlining a process to identify the realpositions and shapes of teeth, which is performed by an image processor;

FIG. 11 is a view explaining a difference between angles from the sameposition in the Z-axis direction on a 3D panoramic image to the X-raytube, which angles are caused by positional changes in the rotationcenter of the pair of the X-ray tube and the detector;

FIG. 12 is a view pictorially showing an example of a 3D referenceimage;

FIG. 13 is a perspective view explaining a plurality of paralleltomographic planes added to a 3D referential tomographic plane;

FIG. 14 is a view explaining differences of positions projected on theplurality of tomographic planes, which positions are obtained when thesame Z-axial position on a 3D panoramic image is projected to the X-raytube, wherein the positional differences are caused by positionalchanges in the rotation center of the pair of the X-ray tube and thedetector;

FIG. 15(1) is a view explaining, together with FIG. 15(2), the processto identify optimally-focused tomographic planes for each of positionson the 3D reference image;

FIG. 15(2) is a view explaining, together with FIG. 15(1), the processto indentify optimally-focused tomographic planes for each of positionson the 3D reference image;

FIG. 16 is a graph exemplifying a frequency analysis result in anidentification process for the optimally focused positions;

FIG. 17 is a graph exemplifying the position of an optimally focusedtomographic plane obtained in the identification process for theoptimally focused positions;

FIG. 18 is a graph exemplifying frequency characteristic patternschanging depending on tomographic plane positions;

FIG. 19 is a view explaining a state where the real positions of teethare deviated from the 3D referential tomographic plane;

FIG. 20 is a view explaining a state where a tooth is shifted from theposition of the 3D referential tomographic image to its real positiondepending on the size of an enlargement factor;

FIG. 21 is a view explaining a state where a tooth is shifted from theposition of the 3D referential tomographic image to its real positiondepending on the size of an enlargement factor;

FIG. 22 is a view explaining a state where a tooth is shifted from theposition of the 3D referential tomographic image to its real positiondepending on the size of an enlargement factor;

FIG. 23 is a perspective view explaining a process for moving processingpoints on the 3D reference image in order to perform the positionindentifying process;

FIG. 24 is a perspective view explaining identification of the positionof an optimally focused tomographic plane being indentified everyprocessing point and its abnormal identification;

FIG. 25 is a view pictorially showing identification of the position ofan optimally focused tomographic plane and a 3D autofocus image producedthrough smoothing;

FIG. 26 is a view explaining a concept of processing for projecting the3D autofocus image onto the 3D referential tomographic plane;

FIG. 27 is a pictorial view explaining an image projected to the 3Dreferential tomographic plane and a ROI set on the image;

FIG. 28 is a view explaining a concept of processing for projecting the3D autofocus image to a two-dimensional plane owned by a referentialpanoramic image;

FIG. 29 is a view pictorially explaining a 2D reference image and a ROIwhich is placed thereon;

FIG. 30 is a view explaining a marker and how to use the marker, whichis described by a modification;

FIG. 31 is a view explaining profiles of frequency characteristics ofvarious types of markers;

FIG. 32 is a view explaining a marker and how to use the marker, whichis description of another modification; and

FIG. 33 is a view explaining a marker and how to use the marker, whichis description of still another modification.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to the accompanying drawings, embodiments of the presentinvention will now be described.

Referring to FIGS. 1-29, an embodiment of a three-dimensional positionidentifying apparatus, a radiation imaging apparatus and an imagingmethod using radiation, which are according to the present invention,will now be described. These apparatuses and method are put intopractice as a dental panoramic imaging apparatus that uses X-rays (X-raybeams). This panoramic imaging apparatus will now be described.

FIG. 1 outlines the appearance of the panoramic imaging apparatus 1.This panoramic imaging apparatus 1 is able to scan, with an X-ray beam,the jaw of an object to obtain digital X-ray transmission data, and usethe data to specify a real position (i.e., actual position) of the toothrow of a three-dimensional structure in the jaw and to produce apanoramic image of the tooth row whose irregularities (or differences)caused due to a later-described enlargement factor are compensated. Inaddition to this basic performance, the panoramic imaging apparatus 1 isable to provide breakthrough features such as various modes of displayand measurements based on such panoramic images. Additionally thisimaging apparatus can reduce an amount of X-ray exposure to objectsbeing imaged and provide operators with good usability. The foregoingbasic performance is obtained by using a tomosynthesis technique.

The configuration of this panoramic imaging apparatus 1 will now beoutlined. As shown in FIG. 1, this apparatus 1 is provided with a gantry11 with which data is acquired from an object (patient) P who is in astanding position, for example, and a control & calculation apparatus 12realized by use of a computer. The control & calculation apparatus 12 isconfigured to control data acquisition performed by the gantary 11,produce panoramic images based on the acquired data, and post-processthe panoramic images in an interactive or manual manner with an operator(doctor or medical operator).

The gantry 11 has a standing unit 13 and an imaging unit 14 movableupward and downward relative to the standing unit 13. The imaging unit14 is attached to the pillar of the standing unit 13 to be movableupward and downward in a predetermined range.

For the sake of easier explanation, the panoramic imaging apparatus isgiven the XYZ orthogonal coordinate system whose Z-axis is assigned tothe longitudinal direction, i.e., the vertical direction, of thestanding unit 13. Incidentally, a two-dimensional panoramic image,described later, is represented with its abscissa axis defined as aj-axis and its ordinate axis defined as an i-axis (i.e, Z-axis).

The imaging unit 14 includes a vertical movement unit 23 whose sideappearance is approximately C-shaped and a rotation unit 24 rotatably(turnably) supported by the vertical movement unit 23. The verticalmovement unit 23 is movable in a given range of height in the Z-axisdirection (longitudinal direction) by a not-shown vertical movementmechanism (for example, a motor and a lack/pinion device) arranged inthe standing unit 13. A command for this movement is provided from thecontrol & calculation apparatus 12 to the vertical movement mechanism.

As described, the vertical movement unit 23 has a side appearance whichis approximately C-shaped, and an upper arm 23A and a lower arm 23Blocated on the upper and lower sides respectively and a longitudinal arm23C integrally connecting the upper and lower arms 23A and 23B. Thelongitudinal arm 23C is movably, in the vertical direction, supported onthe foregoing standing unit 13. Of these arms 23A-23C, the upper arm 23Aand the longitudinal arm 23C cooperatively provide an imaging space(real space). Inside the upper arm 23A, a rotary drive mechanism 30A forrotary drive (for example, an electric motor and a reduction gear) isarranged. The rotary drive mechanism 30A receives a command for rotarydrive from the control & calculation apparatus 12. This rotary drivemechanism 30A has an output shaft, which is the rotation shaft of theelectric motor, arranged to protrude from the upper arm 23A downward(downward in the Z-axis direction). To this rotation shaft, the rotationunit 24 is rotatably coupled. That is, the rotation unit 24 is arrangeddownward from the vertical movement unit 23, and rotates responsively tothe drive of the rotary drive mechanism 30A.

The rotary drive mechanism 30A is linked with a movement mechanism 30B.This movement mechanism 30B is composed of devices such as a not-shownelectric motor and gears. This movement mechanism 30B is also drivenresponsively to a command for rotary drive from the control &calculation apparatus 12, and is capable of moving the rotary drivemechanism 30A, i.e., the rotation unit 24 along the X-Y plane. Hence,the rotation center of a pair of an X-ray tube and a detector, whichwill be described later, can be moved to two-dimensionally trace alater-described trajectory which is along a given orbit in apredetermined range of the X-Y plane.

Meanwhile, the lower arm 23B extends to have a predetermined length inthe same direction as that of the upper arm 23A. A chin rest 25 isplaced on a tip end of the lower arm. A bite block 26 (or simply bite)is detachably attached to the chin rest 25. An object P, i.e., patient,bites the bite block 26, so that the chin rest 25 and the bite block 26provide a function of positioning the oral cavity of the objet P.

The rotation unit 24 has also an approximately C-shaped appearance whenbeing viewed from one side thereof in its used state, where the rotationunit 24 is rotatably attached to the motor output shaft of the upper arm23A, with its opened end side directed downward. Practically, therotation unit 24 has a lateral arm 24A rotatable (turnable) parallelwith the lateral direction, that is, the X-Y plane and right and leftvertical arms (the first and second vertical arms) extending downward(in the Z-axis direction) from both ends of the lateral arm 24A. Thelateral arm 24A and the first and second vertical arms 24B and 24C,which are the right and left arms, are located within the imaging space(the real space), and driven to operate under the control of the control& calculation apparatus 12.

At an inner lower end of the first vertical arm 24B, an X-ray tube 31 isprovided which functions as a radiation emitting source. This X-ray tube31 is for example a rotating anode X-ray tube and has a target (anode)which radially emits X-rays toward the second vertical arm 24C. Thefocus of an electron beam made to collide with the target is as small inradius as 0.5-1 mm, that is, the X-ray tube 31 has a point X-ray source.On the X-ray output side of the X-ray tube 31, there is provided acollimator 33 having a slit. This slit collimates a comparatively thinbeam-shaped X-ray, which is incident to the detector 32, to an actualacquiring window (for example, a window whose width is 5.0 mm) of thedetector 32. Elements that compose the radiation emitting source mayinclude this collimator 33.

In contrast, at an inner lower end of the second vertical arm 24C, thereis provided, as a radiation detecting means, a digital type of X-raydetector 32 equipped with X-ray detection elements two-dimensionallyarrayed (for example, arrayed in a matrix of 64×1500) and produced todetect the incident X-rays through the incidence window. By way ofexample, this detector 32 has a longitudinally-long shaped detectingportion (for example, 6.4 mm width×150 mm long) which is made of CdTe.In the present embodiment, since the tornosynthesis technique isadopted, it is indispensable to provide the detector 32 with a pluralityof X-ray detecting elements in its lateral (width) direction.

The detector 32 is arranged such that its longitudinal direction agreeswith the Z-axis direction. The detector 32 has a lateral effective widthwhich is set to, for example, approximately 5.0 mm by the foregoingcollimator 33. This detector 32 is capable of acquiring digital imagedata in accordance with amounts of incident X-rays at a frame rate of,for example, 300 fps (for example, 64×1500 pixels per frame). Theacquired data are called “frame data.”

During imaging, the X-ray tube 31 and the detector 32 are located to beopposed to each other with the oral cavity of the object P therebetween,and driven to rotate together as paired devices around the oral cavity.However, the rotation is not a rotation to draw a simple circle, and thepair of the X-ray tube 31 and the detector 32 is driven such that therotation center RC of the pair traces a given chevron-shaped orbitconsisting of two connected arcs inside an approximatelyhorseshoe-shaped tooth row, as shown in FIG. 2. This given orbit is anorbit previously designed to enable the X-ray to focus on a sectiontaken along a standard shape and size of the tooth row in the oralcavity (hereinafter, such a section is referred to as a 3D referencetomographic plane SS) and trace the 3D referential tomographic plane SS.When the X-ray is focused to trace the 3D referential tomographic planeSS, the X-ray tube 31 and the detector 32 are not always rotate at anequal angular speed to the 3D referential tomographic plane SS. Thisrotation can be a rotation referred to as “movement along the toothrow”, in which the angular speed is changed arbitrarily during therotation.

It is necessary that the X-ray tube 31 and the detector 32 should bemoved to be opposed to each other and keep the oral cavity of the objectP located between such devices. However this opposed attitude does notalways require the X-ray tube 31 and the detector 32 to be directlyopposed to each other. Depending on how to design the apparatus, theX-ray tube 31 and the detector 32 rotate independently of each otherand, though the oral cavity of the object P should be locatedtherebetween, the X-ray is radiated obliquely to the object.

The 3D referential tomographic plane SS presents an approximatelyhorseshoe-shaped trajectory on the X-Y plane, i.e., when being viewed inthe Z-axis direction, as described, and an example of such a trajectoryis shown in FIG. 2. The trajectory of this 3D referential tomographicplane SS is also known from, for example, a paper “R. Molteni, “Auniversal test phantom for dental panoramic radiography” MedicaMudi,vol. 36, no. 3, 1991.” Information indicative of spatial positionsdefining this 3D referential tomographic plane SS is stored in a ROM 61in advance.

Incidentally, although the 3D referential tomographic plane SS can beset as a known plane, this plane may be set to be according to eachpatient in advance. As such a setting technique, a camera is used toobtain a surface image of an object and the surface image is used toproduce a desired three-dimensional section, madical modalities such asan MRI (magnetic resonance imaging), a CT (computed tomography) scanner,and an ultrasound diagnostic apparatus are used to image a patient'sdesired three-dimensional section, or three-dimensional data of apatient which are imaged by such medical modalities are used to obtain adesired three-dimensional section. Any of such sections can be used as a3D referential tomographic plane. The 3D referential tomographic planeSS may be set using known techniques as above, and data showing theplane may be stored in the ROM 61 in advance.

The X-ray tube 31, the 3D referential tomographic plane SS, the detector32, a rotation axis AXz, and the rotation center RC through which therotation axis AXz passes have a geometrical positional relationshipshown in FIG. 3. The 3D referential tomographic plane SS is parallelwith the incident window of the detector 32 (the window is an X-raydetecting plane Ldet: refer to FIG. 6) and is a section curved along theZ-axis direction. This plane SS is defined as an elongated rectangularsection when being developed two-dimensionally.

FIG. 4 shows an electric block form for control and processing performedby this panoramic imaging apparatus. As shown in the figure, the X-raytube 31 is connected to the control & calculation apparatus 12 via ahigh-voltage generator 41 and a communication line 42. The detector 32is connected to the control & calculation apparatus 12 via acommunication line 43. The high-voltage generator 41 is arranged at thestanding unit 13, the vertical movement unit 23, or the rotation unit 24and responds to a control signal from the control & calculationapparatus 12 to be controlled according to X-ray radiation conditionssuch as a tube current and a tube voltage to the X-ray tube 31 and atiming sequence for the radiation.

The control & calculation apparatus 12 is required to process largeamounts of image data, and is composed of for example a personalcomputer which is capable of storing large amounts of image data. Thecontrol & calculation apparatus 12 includes, as its essentialcomponents, interfaces 51, 52 and 62, a buffer memory 53, an imagememory 54, a frame memory 55, an image processor 56, a controller (CPU)57, and a D/A converter 59, which are mutually communicably connectedvia an internal bus 50. The controller 57 is communicably connected toan operation device 58, and the D/A converter 59 is also connected to amonitor 60. A printer 64 is also communicably connected to the internalbus 50.

Of the above components, the interfaces 51 and 52 are connected to thehigh-voltage generator 41 and the detector 32 respectively, andresponsible for conducting communication on control information andacquired data transmitted from/to the controller 57 and to/from thehigh-voltage generator 41 and detector 32. The other interface 62connects the internal bus 50 and a communication line, which allows thecontroller 57 to be communicable with an external apparatus. It istherefore possible that the controller 57 takes in oral images acquiredby an external oral X-ray imaging apparatus and outputs panoramic imagesacquired by the present apparatus, to an external server based on forexample DICOM (Digital Imaging and Communications in Medicine) protocol.

The buffer memory 53 temporarily stores digital-quantity frame datareceived from the detector 32 via the interface 52.

The image processor 56, which is under the control of the controller 57,has functions of producing panoramic images of a predetermined 3Dreferential tomographic plane provided by the apparatus itself andperforming post-processes to utilize the panoramic images in aniterative manner with an operator. Programs for realizing such functionsare stored in the ROM 61 in advance. Hence, the ROM 61 serves as arecording medium in which programs according to the present inventionare stored. While such programs can be stored in the ROM 61 in advanceas stated above, they can be installed into recording mediums such as anot-shown RAM, via a communication line or a portable memory from anexternal system in some cases.

In the present embodiment, the 3D referential tomographic plane isprepared previously by the apparatus. Alternatively, the 3D referentialtomographic plane may be provided by selecting a desired one from pluraltomographic planes prepared previously by the apparatus beforeperforming imaging. In other words, the 3D referential tomographic planeis a fixed section in the imaging space, but the foregoing selectionallows the plane to be movably positioned in a limited amount of rangein the depth (back-and-forth) direction of a tooth row.

Frame data to be processed or processed now by the image processor 56and image data are stored in the image memory 54 in a readable andwritable manner. The image memory 54 is composed of for example alarge-capacity recording medium such as a hard disc (nonvolatile andreadable and writable). The frame memory 55 is used to display imagedata such as panoramic image data reconstructed and panoramic imagerdata to be post-processed. The image data being stored in the framememory 55 are read at intervals from the D/A converter 59 to beconverted into corresponding analog signals, and displayed on themonitor 60.

The controller 57 controls the operations of all the components of theapparatus based on programs for control and processing, which arepreviously stored in the ROM 61. The programs are set such that thecontroller 57 receives interactively information showing operator'soperations for respective control items. Hence, the controller 57 isable to command acquisition (scanning) of frame data or otheroperations, as will be described later.

As shown in FIG. 1, a patient poses in a standing position or a seatedposition with his or her chin seated on the chin rest 25, his or hermouth biting the bite block 26, and his or her head touched to a headrest 28. This allows the patient's head (jaw) to be fixed positioned inan approximately central part in the space in which the rotation unit 24rotates. With this patient's fixed posture, under the control of thecontroller 57, the rotation unit 24 rotates around the patient's headalong the X-Y plane and/or an oblique plane to the X-Y plane (refer toarrows shown in FIG. 1).

During the rotation, under the control of the controller 57, thehigh-voltage generator 41 supplies to the X-ray tube 31 a pulse-modehigh voltage (designated tube voltage and tube current) at intervals,whereby the X-ray tube 31 is driven on the pulse mode. This allows theX-ray tube 31 to radiate pulsed X-rays at intervals. The X-rays aretransmitted through the patient's jaw (including the tooth row portion)positioned at the designated imaging position) and enters the detector32. Responsively to this, the detector 32 detects the incident X-rays ata very fast frame rate (for example 300 fps) as described, and outputsin sequence, frame by frame, corresponding electric-quantitytwo-dimensional frame data (for example 64×1500 pixels). The outputtedframe data are transmitted to the buffer memory 53 via the communicationline 43 and the interface 52 in the control & calculation apparatus 12for temporal storage therein. The frame data in the buffer memory 53 arethen transferred to the image memory 54 for storage therein.

Hence, the image processor 56 is configured to reconstruct (produce), asa panoramic image (a referential panoramic image), a tomographic imagethat focuses the 3D referential tomographic plane SS using the framedata stored in the image memory 54. That is, this referential panoramicimage is defined as “a panoramic image obtained under an assumption thata tooth row is present at and along the 3D referential tomographic planeSS.” In addition, the image processor uses this referential panoramicimage to produce a three-dimensional (3D) referential image and a threedimensional (3D) autofocus image. This processing is outlined in FIG. 5.The 3D referential image is defined as “a three-dimensional imageobtained under an assumption that a tooth row is present at and long the3D referential tomographic plane SS.” The 3D autofocus image is definedas “a surface image (i.e., a pseudo 3D surface image) automaticallyoptimally focusing on the tooth row from the 3D referential image usingframe data or data of the referential panoramic image.” In other words,the 3D autofocus image is a surface image with less blur and optimallyfocused, where the real position and actual size of a tooth row isdepicted with higher precision.

In particular, it can be said that the 3D autofocus image takes intoconsideration that 3D autofocus images of individual persons differperson by person in most cases. In practice, it is very rare to findthat tooth rows of individual persons being imaged are at and along the3D referential tomographic plane SS (refer to FIG. 6). The tooth rowsmay partially or entirely offset from the 3D referential tomographicplane SS or may be oblique to that plane. In light of this, the 3Dautofocus image is produced by automatically and accurately identifyingthe actual three-dimensional spatial position and shape of the tooth rowof each person being imaged and automatically extracting from theidentified results the actual shape of the person's tooth row.

The X-rays radiated from the X-ray tube 31 (serving as a point X-raysource) are transmitted through the oral cavity of an object P, and thenare detected by the long detector 32 having a certain length in theX-axis direction. Hence, the radiated directions of the X-rays areoblique, as shown in FIGS. 3 and 6. Accordingly, there exists a ratiobetween the actual size of the tooth and the size of an image producedby the shade of the tooth projected onto the X-ray incident surface Ldetof the detector 32, and this ratio changes depending on positions of therotation center RC. In the present embodiment, this ratio is called “anenlargement factor.” In the example shown in FIG. 6 (only the height ofthe tooth is explained), a ratio between the actual height P1 _(real) ofthe tooth and the height P1 _(det) on the X-ray incident surface Ldetchanges depending on positions of the rotation center RC. As exemplifiedin FIG. 2, the rotation center RC takes various positions, which changeduring one scan (data acquisition). An orbit on which the positionstrace is set previously. The reason for the orbit is follows. As shownin FIG. 6, a distance DaII between the X-ray tube 31 and the detector 32is kept unchanged, and distances D1 and D2 from the rotation center RCto the X-ray tube 31 and the detector 32 respectively are also keptunchanged. Meanwhile, for scanning with focusing on the 3D referentialtomographic plane SS, design is made such that, during one scan, theorbit traced by the positions of the rotation center RC changes todepict, by way of example, the shape of a mountain (refer to FIG. 2) tothe horseshoe-shaped curved tooth row.

Concretely, a distance D3 from the rotation center RC to the 3Dreferential tomographic plane SS and a distance D4 (D3+D4=D2) from thedetector 32 to the 3D referential tomographic plane SS change dependingon advancement of the scanning. Depending on these changes, the rotationcenter RC comes closer to recedes from the tooth row, so that the X-raytube 31 comes closer to or recedes from the tooth row as well. Since theX-ray tube 31 has an X-ray source which can be regarded as a pointsource, the size of a projection image onto the detection surface Ldetbecomes bigger as the X-ray tube 31 comes closer to the tooth row evenunder a condition where the height of the tooth is the same. That is,the enlargement factor becomes larger in such a case. In the exampleshown in FIG. 2, compared with a case where the molar teeth are scanned,scanning the anterior teeth allows the rotation center RC to come closerto the tooth row, providing the enlargement factor with larger amountsdepending on how much closer to the tooth row. For example, in the caseof FIG. 2, the distance d1 is given when one of the anterior teeth isscanned in an X-ray radiation direction of 0 degrees for example.Meanwhile, in the case of scanning some of the molar teeth, there areprovided distances d2 and d3 in the X-ray radiation directions of 60degrees and 75 degrees respectively, for example. In this example, thereare provided relationships of d1<d2, d1<d3, and d2<d3. Although thetrajectory of the rotation center RC shown in FIG. 2 is simply anexample, it is always true that the rotation center RC comes closer toand then gets away from the tooth row when scanning is made to focus onthe 3D referential tomographic plane SS by the panoramic imagingapparatus.

In such a case, the enlargement factor changes depending on what part ofthe tooth row is scanned. This fact becomes a significant barrier in anattempt at quantitatively analyzing changes in the structure and/or thetemporal changes of the oral cavity.

In addition, though the above issue about the enlargement factor hasbeen described on an assumption that the tooth row is present along the3D referential tomographic plane SS, it is almost certain that such anassumption is not true. In effect, it is mostly correct that patients'tooth rows are shifted entirely or partially from the 3D referentialtomographic plane SS. Thus the imaging should consider this fact.

The conventional panoramic images are produced with no consideration ofthe issues due to changes in the foregoing enlargement factor and shiftsof tooth rows from the 3D referential tomographic plane SS. Thus, it isvery difficult to quantitatively analyze the structure from theconventional panoramic images. In this regard, it is desired to providea panoramic imaging apparatus capable of imaging objects with accuracyeven when tooth rows are different in shapes and/or positions everyobject, and/or regardless of what part of the same object′ tooth row isimaged.

With consideration this, the panoramic imaging apparatus according tothe present embodiment has a feature that image distortion due todifferences in the enlargement factor even for the same tooth row can beremoved, part by part, and it is possible to automatically andaccurately identify a three-dimensional spatial real position (includinga shape) of a patient′ tooth row. Thus it is possible to providethree-dimensional panoramic images with higher identification accuracyof positions (shapes), than has been provided in the past.

In the present embodiment, a tornosynthesis technique (simply calledtornosynthesis) is used to obtain images of tomographic planes orsections of an object. Practically, of frame data (sets of pixel data)acquired at intervals by scanning, a plurality of frame data relating toindividual positions of a trajectory of the 3D referential tomographicplane obtained by projecting the 3D referential tomographic plane to theX-Y plane are selected. Such selected frame data are shifted to beoverlapped with each other depending on their positions and added witheach other (shift & add). Hence, an “optimal focus” referred to in thepresent embodiment means that “being best focused, being lessdefocused”, which also means that a region of interest in an image ishigher in resolution than other regions thereof or an entire image has ahigher degree of resolution.

When a referential panoramic image is produced, data composing thisimage are stored in the image memory 54 and also displayed by themonitor 60 in an appropriate display mode. The display mode is decidedby an operator's intention which is given through the operation device58.

(Image Processing)

Referring to FIG. 5, processing performed cooperatively by thecontroller 57 and the image processor 56 will now be described. Thisprocessing includes, as described before, data acquisition by scanning,reconstruction of a referential panoramic image which is a pre-process,and production of 3D autofocus image (surface image) which is a mainprocess, and display and/or measurement based on various modes which usethe 3D autofocus images.

<Data Acquisition and Reconstruction of Referential Panoramic Image>

First, the controller 57 reads positional information of a 3Dreferential tomographic plane SS from the ROM 61 (step S0), afterpreparations such as positioning an object P for the imaging. This 3Dreferential tomographic plane SS may be a section decided in astatistical manner or a section previously set for each object.

The controller 57 then responds to an operator's command given throughthe operation device 58 to command scanning for acquiring data (StepS1). By this command, the rotary drive mechanism 30A, the movementmechanism 30B, and the high-voltage generator 41 start to be drivenaccording to a predetermined control sequence. As a result, duringrotation of the pair of the X-ray tube 31 and the detector 32 around thejaw of the object P, the X-ray tube 31 radiates a pulsed (or continuouswave) X-ray at intervals (or continuously). As described before, thepair of the X-ray tube 31 and the detector 32 is driven to rotate undera given drive condition so as to optimally focus on the 3D referentialtomographic plane SS (refer to FIG. 6). The X-rays radiated from theX-ray tube 31 are transmitted through the object P to be detected by thedetector 32. Accordingly, the detector 32 outputs, for example, at arate of 300 fps, digital-quantity frame data (i.e., pixel data) in whichamounts of X-ray transmission are reflected. The outputted frame dataare temporarily stored in the buffer memory 53.

After the command for the scanning, the next command for the processingis provided to the image processor 56. The image processor 56reconstructs a referential panoramic image PIst based on the shift & addprocess based on the tornosynthesis technique according to the spatialpositions of the 3D referential tomographic plane SS, and the respectivepixel values of the reconstructed image are stored (step S2). In thisreconstruction process, the reconstructed image is multiplied bycoefficients such that, similarly to the conventional situationlongitudinal and lateral enlargement factors at the center of theanterior teeth become equal to each other.

Although how to reconstruct an image is known, this will now bedescribed a little. A set of frame data used for the reconstruction isobtained from a mapping characteristic which shows, as shown in FIG. 7for example, mapping positions in the lateral direction of a panoramicimage and a set of frame data which will be subjected to mutual additionfor producing an image at the mapping positions. A curve showing thismapping characteristic consists of two curved portions which are steeperdepending on molar teeth on both sides in the frame data direction(abscissa axis) and a curbed portion which is gentler than those of thetwo curved portions for the molar teeth depending on the anterior teeth.As shown, in this projection characteristic is used to designate adesired mapping position in the lateral direction of the panoramicimage. Based on this designation, the set of frame data used to producean image at the designated mapping position and amounts of shift (i.e.degrees of overlapping necessary frame data, which is a gradient of thecurve) are designated. The designated frame data (i.e, pixel values) areshifted in accordance with the designated shift amounts to be overlappedon one another and added to each other, thus providing data of alongitudinally extending image at the designated mapping position. Byrepeating the designation of mapping positions and shift & addcalculation through the entire range in the lateral direction of thepanoramic image, it is possible to reconstruct the referential panoramicimage PIst which focuses on the 3D referential tomographic plane SS.

The image processor 56 then displays the constructed referentialpanoramic image PIst on the monitor 60 (step S3), of which example ispictorially shown in FIG. 8

Since the referential panoramic image PIst is an image produced byshifting the frame data to be overlapped on one another and mutuallyadding them, this image is two-dimensionally rectangular. Longitudinaland lateral distortion at the anterior teeth in this image PIst, whichis due to a difference of the enlargement factor in the longitudinal andlateral direction, is improved to some extent similarly to theconventional, because the image is multiplied by the coefficients so asto make the longitudinal and lateral enlargement factors equal to eachother at the center of the anterior teeth. However, as advancing to andthrough the molar teeth, the longitudinal and lateral ratios of teethbecome shifted from the correct ones. That is, the molar teeth aredepicted to be less in size than the real size thereof. In manyconventional cases, doctors are obliged to tolerate such panoramicimages with distortion.

<Setting ROI on Referential Panoramic Image>

Then, the image processor 56 determines whether or not the operator usesthe operation device 58 to set a ROI (a region of interest) on thereferential panoramic image PIst (step S4). For example, the ROI shows arectangular partial region in which the interpreter has a specialinterest. The ROI is not always limited to rectangular. In addition, theROI can be set on the panoramic image automatically focused, which willbe described later.

When the determination at step S4 is YES, the image processor 56responds to operational information from the operator to set the ROI onthe referential panoramic image PIst (step S5). Then a partial image,which corresponds to the partial region sectioned by the ROI, is clippedout, and the partial image is displayed in a magnifying scale forexample (step S6). For example, as shown in FIG. 9, this partial imageis displayed in a superposed manner on the original referentialpanoramic image PIst. Alternatively, one or more partial images can bedisplayed by being mapped in a template in which blocks arranged topictorially depict partial tooth rows in both the upper and lower teethare arranged in a predetermined order.

Then the image processor 56 determines whether or not the processingshould be ended. This determination depends whether or not there isoperational information from the operator (step S7). When it isdetermined to continue the processing (NO at step S7), the processing isreturned to step S4 for repetition of the foregoing steps. In contrast,when the determination shows completion of the processing, theprocessing shown in FIG. 5 is ended.

Meanwhile, when the determination at step S4 is NO, that is, when theROI will not be set, the image processor 56 proceeds to the next step.Practically, it is determined based on operational information from theoperator whether or not production of a 3D autofocus image is performedas a main process (step S8). If it is determined that this productionwill not be performed (NO at step S8), the processing is made to returnto step S7 to determine ending the processing similarly as described.

<Specification of Position of Optimally Focused Section>

In contrast, when it is determined that production of the 3D autofocusimage is desired (YES at step S8), the processing proceeds to asubroutine provided at step S9. The processing executed at step S9provides one of the features of the present invention, which isautomatic identification of the real position and shape of a tooth row.In the identification, distortion in sizes of teeth is corrected, whichdistortion is due to the X-ray radiation directions oblique to theZ-axis direction

FIG. 10 shows processing of the subroutine for such identification.

First, the image processor 56 considers the X-ray radiation directionsto produce an image along the 3D referential tomographic plane SS (StepS51). Concretely the referential panoramic image PIst (rectangular) iscoordinate-converted to a curved plane parallel with the 3D referentialtomographic plane SS (a curved plane) to produce a 3D referential imagealong the coordinate-converted curved plane. Each of the pixels of thethis 3D referential image is projected to the 3D referential tomographicplane SS along each of the X-ray radiation directions DRx by obtainingframe data by calculating changes of tomographic planes andcoordinate-converting the obtained frame data. This provides aprojection image along the curved 3D referential tomographic plane SS.The pixel values of this projection image are stored in the image memory54.

The projection is performed, as shown in FIG. 11, along each of theoblique projection directions directed to the rotation center RC (RC1,RC2), that is, the position where the X-ray tube 31 is present. Whenreferring to the example in FIG. 11, a pixel located at a position Pn inthe height direction (the Z-axis direction) of the 3D panoramic imagewill be projected to different positions SS1 and SS2 on the image of the3D referential tomographic image SS, which is due to differences ofpositions at each of which the X-ray tube 31 is located.

The projection image produced by this projection is called a 3Dreferential image PIref in the present embodiment. This 3D referentialimage PIref is produced by oblique projection with consideration ofcharacteristics of the foregoing enlargement factor, in which theoblique projection is performed at each of the pixels of the referentialpanoramic image PIst. By this oblique projection, enlargement of teethbelonging to the anterior teeth, which have large enlargement factors,is corrected to have real sizes thereof, while enlargement of teethbelonging to the molar teeth on both sides of the tooth row, which havesmall enlargement factors, is also corrected to have real sizes thereof.Hence, in the 3D referential image PIref, the teeth are depicted withtheir real sizes and have no or less distortion which is due to the sizeof the enlargement factors caused by the moved rotation center RC duringthe scanning. However it should be noted that this 3D referential imagePIref is produced on the assumption that the tooth row is present at andalong the 3D referential tomographic plane SS. It is rare that actualteeth are present at and along the plane SS, so that it is required toperform further processing to identify the real spatial positions andshapes of the teeth.

The image processor 56 displays the 3D referential image PIref on themonitor for operator's reference (step S52). This is shown in FIG. 12.

The image processor 56 then adds a plurality of curved and paralleltomographic planes to the 3D referential tomographic plane SS (stepS53). This is shown in FIG. 13. As shown, a plurality of tomographicplanes are added to the 3D referential tomographic plane SS such thatsuch tomographic planes are set before and after the plane SSrespectively in the X-ray radiation directions DRx (i.e., the depthdirection of the tooth row). By way of example, plural tomographicplanes SFm-SF1 are located on the front side of the 3D referentialtomographic plane SS at intervals of D1 (for example, 0.5 mm), whileplural tomographic planes SR1-SRn are located on the rear side of theplane SS at intervals of D2 (for example, 0.5 mm). The intervals D1 andD2 may be equal to each other or different from each other. In addition,the number of tomographic planes to be added may be one on the front andrear sides of the plane SS respectively (i.e., m, n=1) or may be one orplural on either the front side or the rear side of the plane SS.

Incidentally, position data indicative of the virtually addedtomographic planes SFm-SF1 and SR1-SRn are previously stored in the ROM61 together with positional data of the 3D referential tomographic planeSS, so that the image processor 56 can perform the addition throughreading of the positional data and loading them into a work area of theimage processor 56. The heights of the tomographic planes SFm-SF1, SS,and SR1-SRn are decided appropriately in consideration of the maximumgradient of the X-ray radiation directions DRx and the height of thetooth row. Every time the identification processing is performed, thepositions (the intervals D1, D2) of the tomographic planes to be addedand the number thereof may be changed interactively.

Then, similarly to the process at step S51, with consideration of theangles of the X-ray radiation directions DRx, the image processor 56projects the referential panoramic image PIst onto each of thetomographic planes SFm-SF1 and SR1-SRn by obtaining frame data throughcalculation of changes of tomographic planes and coordinate-changing theobtained frame data (step S54). As a result, images projected to therespective added tomographic planes SFm-SF1 and SR1-SRn are produced.The pixel values of such projection images are stored in the imagememory 54.

In the present embodiment, the produced projection images are referredto as 3D added images PIsfm, . . . , PIsf1, PIsr1, . . . , PIsrn. Eachof these 3D added images PIsfm, . . . , PIsf1, PIsr1, . . . , PIsrn isalso produced by the oblique projections performed through theindividual pixel positions of the referential panoramic image PIst, inwhich the oblique projections take into account of the foregoingdifferences in the enlargement factors. This is exemplified in FIG. 14,in which the same pixel existing at a position Pn in the heightdirection of a 3D panoramic image (i.e., the Z-axis direction) isprojected onto different positions on the respective 3D added imagesPIsfm, . . . , PIsf1, PIsr1, . . . , PIsrn, which are due to differencesin positions where the X-ray tube 31 is located.

Hence, the teeth depicted in the 3D added images PIsfm, . . . , PIsf1,PIsr1, . . . , PIsrn are depicted with their real sizes and distortiondue to the variation of the enlargement factors, which is due to themovement of the rotation center RC during the scanning, is removed orsuppressed from such 3D added images. It should be noted however thatthe 3D added images PIsfm, . . . , PIsf1, PIsr1, . . . , PIsrn areproduced on the assumption that the tooth row is present at and alongeach of the 3D added images PIsfm, . . . , PIsf1, PIsr1, . . . , PIsrn.

As a modification, the plural 3D added images PIsfm, . . . , PIsf1,PIsr1, . . . , PIsrn thus produced can be displayed on the monitor 60 asthree-dimensional images as they are or displayed on the monitor 60 asrectangular two-dimensional images produced through coordinateconversion.

The image processor 56 then designates an initial position P(x, y, z) onthe 3D referential image PIref, that is, the 3D referential tomographicplane SS (step S55; refer to FIG. 15(A)). After this, the imageprocessor designates a line segment Lc of a given length centering thedesignated position P(x, y, z) on the 3D referential image PIref (stepS56; refer to FIG. 15 (B)). This line segment Lc has the given lengthcorresponding to 2 n pieces (n=1, 2, 3, . . . ; for example 128 pieces).As modifications, the line segment Lc can be drawn along a part of thecurved 3D referential tomographic plane SS so that the line segment iscurved or can be drawn in a limited range regarded as being linear.

Then the image processor 56 virtually adds plural line segments Ladd onthe upper and lower sides of the designated line segment Lc(x, y, z) onthe image, respectively, in which the plural line segments Ladd have thesame length as that of the line segment Lc(x, y, z) (step S57; refer toFIG. 15(C)).

The image processor 56 then reads, from the image memory 54, the pixelvalues Pij of respective 2n-piece pixels composing each of the foregoingline segment Lc and plural line segments Ladd, and assigns the readpixel values to the respective line segments (step S58). The pixelvalues Pij have been already acquired and stored through the foregoingsteps S51 and S54.

The image processor then mutually add the pixel values Pij of the pixelscorresponding to the line segment Lc and line segments Ladd to obtain2n-piece pixel values Pij* that composes the line segment Lc(x, y, z),the 2n-piece pixel values Pij* being for a frequency analysis (step S59;refer to FIG. 15(D)). This addition makes it possible to reducestatistical noise in a later-described frequency analysis of changes inthe pixel values, even if the pixel values of the line segment L(x, y,z) originally contain the statistical noise.

Then, on each of the 3D added images PIsfm, . . . , PIsf1 and PIsr1, . .. , PIsrn, the image processor 56 calculates the spatial positions ofthe line segments Lfm-Lf1 and Lr1-Lrn that face the line segment Lc(x,y, z) designated currently on the foregoing 3D referential image PIref,in the X-ray radiation direction DRx passing through the currentlydesignated position P(x, y, z) (step S60; refer to FIG. 15 (E)). In thiscase, the current center position P(x, y, z) and the length of the linesegment Lc, and the rotational positions of the X-ray tube 31 during thescanning are known. Hence, it is possible to calculate an X-rayradiation range RA which is fan-shaped when being viewed in the Z-axisdirection, in which this range RA is formed by connecting each of bothends of the line segment Lc to the X-ray tube 31. As a result, as longas the position P(x, y, z) is designated, the spatial positions of theline segments Lfm, . . . , Lf1 and Lr1, . . . , Lrn limited by the X-rayradiation range in compliance with the designated position can also bedesignated by the image processor.

The process of step S60 to designate the position the position P(x, y,z) on the 3D referential image PIref is repeated until the same processfor all the positions thereon is completed. Hence, in terms of effectiveX-ray radiation, the X-rays radiated from the X-ray tube 31 whoseposition comes near and farer transmit through the virtually settomographic planes SFm-SF1, SS, and SR1-SRn within a range of H1 to H2(a Z-axial range) in the fan shape (refer to FIG. 15 (F)). Withconsideration this fact, the tomographic planes SFm-SF1, SS and SR1-SRnthemselves may be horseshoe-shaped sections having heights which changein every scanning direction and being parallel with each other.

When the line segments Lfm-Lf1 and Lr1-Lrn have been set as above, theimage processor 56 reads pixel values Pij* of such line segments fromthe image memory 54 (step S61).

As shown in FIG. 15(E), since the X-ray tube 31 has a point X-raysource, the X-ray radiation range RA becomes a fan shape (when beingviewed along the Z-axis direction). Hence, the pixels of each of theline segments Lfm-Lf1 and Lr1-Lrn are not 2^(n)-pieces in number. Thusthe image processor 56 multiplies each of the line segments Lfm-Lf1 andLr1-Lrn by a coefficient depending on the distances D1 and D2 (refer toFIG. 6) in such a manner that the number of pixels of each of the addedline segments Lfm-Lf1 and Lr1-Lrn becomes equal to the number of pixels,2^(n), of the referential line segment Lc(x, y, z). Accordingly, aspictorially shown in FIG. 15(G), all the line segments Lfm-Lf1, Lc andLr1-Lrn are formed to be parallel with each other and to have the samenumber of pixels, 2^(n) (step S62).

After this, the image processor 56 applies a frequency analysis tochanges in the values of pixels of each of all the line segmentsLf1-Lfm, Lc, and Lr1-Lrn (step S63). Thus, as shown in FIG. 15(H), as tothe line segments Lfm-Lf1, L and Lr1-Lrn, there are provided analyzedresults consisting of an abscissa axis showing frequencies and avertical axis showing Fourier coefficients (amplitudes).

In the present embodiment, the frequency analysis is performed usingfast Fourier transformation, but wavelet transformation may be adoptedas such frequency analysis. Moreover, instead of such frequencyanalysis, a sobel filter to calculate the first derivation for edgeextraction can be used for the equivalent process to the above. In usingthis filter, the position of a tomographic plane which provides an edgewith a maximum filtered value can be regarded as an optimally focusedposition.

The image processor 56 then removes noise from the frequency analyzedresults for all the line segments Lf1-Lfm, Lc, and Lr1-Lrn (step S64).FIG. 16 exemplifies the frequency analyzed characteristic for one linesegment. Coefficients of frequency components which belong to a givenfrequency range next to a maximum frequency of the analysis are removed,with coefficients of the remaining high-frequency components adopted.The reason for such removal is that frequency components which arepresent in the given frequency range next to the maximum frequencyprovide noise signal components.

Further the image processor 56 calculates sums of squares for thecoefficients of the frequency analyzed characteristic of each of theline segments, and produces a profile having a vertical axis to whichthe values of sums of squares are assigned and an abscissa axis to whichthe positions of the respective tomographic planes SFm-SF1, SS, andSR1-SRn are assigned, where the X-ray radiation direction DRx passingthrough the initial position P(x, y, z)=P(0, 0, 0) passes through thepositions of such tomographic planes (step S65). This profile isexemplified in FIG. 17. In this profile, the positions of thetomographic planes mean the positions the plural tomographic planesSF1-SFm, SS, and FR1-FRn in the X-ray radiation direction DRx (i.e., thedepth direction of the tooth row).

FIG. 18 exemplifies, as typical patterns, a plurality of types ofprofiles PR1, PR2, PR3 and PR4 respectively showing substances composedof enamel, cancellous bone, air and bite blocks. If there is a substanceof enamel, i.e., tooth, anywhere in the X-ray radiation direction DRxthat passes through the currently designated position P(x, y, z), theprofile PR1 has a sharp peak. If there is cancellous bone in that X-rayradiation direction DRx, the profile PR2 has a gentle convex curve.Similarly, if there exists only air in that X-ray radiation directionDRx, the profile PR3 tends to show a curve having no specific peaks.Moreover, when the bite block is present in that X-ray radiationdirection DRx, the profile PR4 exhibits two sharp peaks. Of such twopeaks, one appearing inward (on the X-ray tube side) in the X-rayradiation direction DRx shows that the substance thereat is enamel,while the other appearing outward (on the detector side) shows that thesubstance thereat is the bite block. Data indicating the patterns of theprofiles shown in FIG. 18 are previously stored, as reference profiles,in the ROM 61 in the form of a reference table.

Hence, the image processor 56 refers to the reference table to specifyan optimum focused position of the tooth in the X-ray radiationdirection DRx passing through the currently designated position P(x, y,z) (step S66).

That is, a pattern recognition technique is used to determine that theprofile obtained in the last step S65 corresponds to which of thereference profiles PR1-PR4. First, when the obtained profile is thereference profile PR2 or PR4, such a profile is withdrawn from theconsideration. In contrast, when the obtained profile corresponds to thereference profile PR1 (i.e., enamel), it is identified that the sectionposition showing its peak, i.e., the position of any of the pluraltomographic planes SF1-SFm, SS, FR1-FRn, is optimally focused. Moreover,when the obtained profile is fit to the reference profile PR4, it isalso identified that an inward sectional position expressing a peak (asectional position showing enamel on the X-ray tube side), in otherwords, the position of any of the plural tomographic planes SF1-SFm, SS,FR1-FRn, is optimally focused.

By the foregoing specifying steps, it is indentified that a portion ofthe tooth depicted at the currently designated position P(x, y, z) isactually present at which position in the depth direction. In effect, atooth portion depicted on the 3D referential image PIref along the 3Dreferential tomographic plane SS may be present on the front or rearsides of the plane SS. The real position of the tooth portion in theimaging space is specified precisely by the foregoing specifying steps.In other words, it can be understood that a tooth portion depicted onthe 3D referential image PIref under the condition that the toothportion is on and along the 3D referential tomographic plane SS isshifted to its real spatial position by the foregoing specifying steps.

As a result, as shown in FIGS. 19-22, every time the position P(x, y, z)is designated, a position P1 on the 3D referential tomographic plane SS(i.e., in the 3D referential image PIref) is shifted to a positionP1real (or a position P2 is shifted to a position P2real). Inparticular, the positions of the line segments Lfm-Lf1 and Lr1-Lrn whichare set on the plural added tomographic planes SFm-SF1 and FR1-FRn areset in consideration of the oblique angle θ of each of the X-rayradiation directions DRx. Thus, the shifted position P1real in the caseof a smaller oblique angle θ (such as θ₁, θ₁′: refer to FIGS. 20(A) and21(A)) becomes lower in the Z-axis direction than in the case of alarger oblique angle θ (such as θ₂, θ₂′: refer to FIGS. 20(B) and20(B)).

Accordingly, the shifted position P1real can be compensated indistortion depending on the oblique X-ray radiation angle θ, i.e., thesize of the enlargement factor. Incidentally as shown in FIG. 22, whenthe tooth is present at and along the 3D referential tomographic planeSS, a relationship of P1=P1real is met. In this case, the 3D referentialtomographic plane SS, at and along which it is assumed that the tooth ispresent, provides a real position, thus providing a shift amount ofzero.

The image processor 56 then proceeds to step S66, at which dataindicating the real position of the tooth portion is stored everyposition P(x, y, z) in the work area thereof.

In this way, as to the currently designated position P(x, y, z) on the3D referential image PIref (i.e., the 3D referential tomographic planeSS), practically, as to the first designed initial position P(0, 0, 0),a specifying process is performed in the depth direction passing throughthe initial position P(0, 0, 0). As the specifying process, filtering isperformed to check whether or not there is a portion of the tooth(enamel). And when it is checked that there is such a tooth portion, anoptimally focused position for the tooth part is specified in the depthdirection.

After this, as shown in FIG. 23 for example, the image processor 56determines whether or not the foregoing specifying steps have beencompleted at all determination points (sampling points) P previously seton the 3D referential image PIref (step S67). This determination isexecuted by checking whether or not the currently processed positionP(x, y, z) is the last position P(p, q, r). If this determination is NOwhich shows the specifying steps have not been completed at all thedetermination points P, the image processor 56 shifts the determinationpoint P(x, y, z) by one (step S68), before returning to the foregoingstep S55 to repeat the foregoing series of specifying steps.

As shown in FIG. 23, the plural determination points P are previouslymapped with given intervals on the 3D referential image PIref (i.e., the3D referential tomographic plane SS). In the example shown here, alongboth the vertical direction i and the horizontal direction j of the 3Dreferential image PIref, the positions P are mapped at the same givenintervals d in the vertical and horizontal directions. Alternatively,the internals d may be differentiated between the vertical andhorizontal directions i and j from each other. The direction along whichthe shift is carried out at step S68 may be any of the vertical,horizontal and oblique directions on the 3D referential image PIref. Asshown in FIG. 23, the shift can be made along the first verticaldirection i, and the line is transferred in the horizontal direction jto repeat the shift in the second vertical direction i. This shiftingmanner is repeated regularly (refer to a reference numeral SC in thefigure). Oppositely to this, the shift can be made along the horizontaldirection j, and the line is transferred in the vertical direction i torepeat the shift in the horizontal direction. Moreover, the shift may beperformed obliquely.

Meanwhile, when the foregoing specifying steps have completed for allthe plural determination points P, the determination at step S67 revealsYES during the repeated processing. This means that, every determinationpoint P, an optimally focused sectional position has been detected inthe depth direction passing through the position P on the 3D referentialtomographic plane SS (including determination whether or not there is anoptimally focused position). In this case, the processing proceeds to aconnection process of the optically focused sectional positions.

<Process to Connect Optimally Focused Sectional Positions>

When it is determined YES at the foregoing step S67, the image processor56 reads data indicative of the optimally focused sectional positionsspecified and stored at step S66 (step S69). The data of these sectionalpositions show positions in each of the X-ray radiation directions DRxpassing through each of the determination points P(x, y, z). This ispictorially exemplified in FIG. 24. In this drawing, filled circlesindicate determination points P(x, y, z) on the 3D referential imagePIref (i.e., the 3D referential tomographic plane SS). In this case, thevertical and horizontal directions of the curved 3D referential imagePIref are denoted by (i, j). In FIG. 24, as shown by open circles, forexample, an optimally focused sectional position at a determinationpoint P(x00, y00, z00) for i, j=0, 0 is read as a position actuallypreset at the position of a tomographic plane SR1, which is shiftedinwardly by one section (toward the X-ray tube side). An optimallyfocused sectional position at a determination point P(x01, y01, z01) fori, j=0, 1, which is next to the determination point for i, j=0, 0, isread as a position also actually present at the position of thetomographic plane SR2, which is shifted inwardly by one plane.Furthermore, an optimally focused sectional position at a determinationpoint P(x02, y02, z02) for i, j=0, 2, which is next to the determinationpoint for i, j=0, 1, is read as a position actually present at theposition of a tomographic plane SR3, which is shifted inwardly by oneplane further. In FIG. 24, for the sake of an easier understanding theprocess at step S68, only one position in the Z-axis direction (i.e.,the vertical direction) are shown, but, at each of the positions in theZ-axis direction, the process at step S68 is performed.

The image processor 56 then performs removal of noise (step S70). In theexample shown in FIG. 24, an optimally focused sectional position at adetermination point P(x03, y03, z03) for i, j=0, 3 is read as a positionactually present at the position of a tomographic plane SFm, which isshifted outwardly by no less than m-piece sections (toward the detectorside). In such a case, the image processor 56 applies, for example, athreshold checking to a difference between the sectional positions tofind out noise, thus regarding it abnormal data. Hence, the data ofmutually adjacent sections are for example smoothed to smoothly connectthe sections and replaced with a new set of smoothed positional data.Alternatively, data used for the data replacement may be produced byselectively giving priority to sectional data closer to the outside ofthe detector. As another alternative, instead of such compensation usingthe data replacement, abnormal data may simply be removed from databeing processed. It is also possible that the abnormal data to beremoved include data abnormal in the Z-axis direction.

The image processor 56 then connects the positions with noise removed(that is, the positions showing the enamel) and three-dimensionallysmoothes the connected positional data, whereby a surface image tracingthe enamel is produced (step S71). Furthermore, the image processor 56displays the produced surface image, as a 3D autofocus image PIfocuswhich is a three-dimensional panoramic image all portions of which areautomatically optimally focused, on the monitor 60 at a desired viewangle (step S72).

Hence, as shown in FIG. 25, it is possible to provide the 3D autofocusimage PIfocus at the desired view angle, where the 3D autofocus imagestructurally shows the tooth row of the oral cavity of the object P inthe clearest manner. In the drawing, the curved horseshoe-shaped range Sshows a range to display the 3D autofocus image PIfocus and solid linesshow the real positions and shapes of the tooth row. As shown by an A-A′line and a B-B′ line, portions such as part of the gum (alveolar bone),mandibular antra, the articulation of jaw, and the carotid artery can bedepicted by a tomographic plane produced apart a given distance from theedges of the teeth (mainly the enamel) and the tomographic plane isprojected three-dimensionally. In such a case, although it is notguaranteed that such portions other than the teeth are optimallyfocused, there will not be an unnatural feeling to the 3D panoramicimages. Of course, the calculation for optimal focusing may be improvedto optimally focus on such portions other than the teeth in the wholecalculation, depending on purposes of diagnosis.

In this way, the produced 3D autofocus image PIfocus is entirely curvedto trace the tooth row and its surface is rough. This “roughness”depicts the real position and shape (contour) of each of the teeth bydensities of pixel values. The remaining parts can also be depicted withno unnatural feeling.

Hence, the autofocus image PIfocus indicating the real position andshape of the tooth row of each object P.

<Various Display Processes>

The image processor 56 then provides the operator with chances toobserve the produced 3D autofocus image PIfocus in other modes.Practically, in response to operation information from the operator, theimage processor 56 determines whether or not it is desired to displaythe 3D autofocus image PIfocus in other modes, in an interactive manner.

By way of example, the image processor 56 determines whether or not itis desired to observe a partial region of the 3D autofocus image (3Dpanoramic image) PIfocus (in FIG. 5, step S10). When the determinationat this step S10 is YES, it is further determined based on informationfrom the operator whether the partial region is desired to be observedusing a 3D referential tomographic plane SS or the rectangular plane(two-dimensional) of a referential panoramic image (step S11). When itis determined at this step S11 that the 3D referential tomographic planeSS is used, the image processor 56 re-projects the 3D autofocus imagePIfocus to the 3D referential tomographic plane SS in the X-rayradiation directions DRx passing through the pixels of the plane SSrespectively (step S12). This re-projection is shown in FIG. 26. Thisre-transmission can be performed by a subpixel method, by which eachpixel of the 3D referential tomographic plane is sectioned by subpixelscorresponding to the three-dimensional pixels and then subjected to there-projection.

A re-projected image to the 3D referential tomographic plane SS isdisplayed on the monitor 60 as a 3D reference image PIproj-3D (stepS13). An example of this 3D reference image PIproj-3D is shown in FIG.27.

Meanwhile, when it is determined at step S11 that the rectangular planeof the referential panoramic image PIst is desired, the image processor56 re-projects the 3D autofocus image PIfocus to the rectangular plane,that is, the plane of a referential panoramic image (step S14). Thisre-projection is also performed by the known subpixel method, by whicheach pixel of the plane of the referential panoramic image is sectionedby subpixels corresponding to the three-dimensional pixels and thensubjected to the re-projection. This re-projection is conceptually shownin FIG. 28. This re-projected image is displayed as a 2D reference imagePIproj-2D on the monitor (step S15). An example of this 2D referenceimage PIproj-2D is shown in FIG. 29.

The operator sets a desired ROI (region of interest) of, for example, arectangle on either the 3D reference image PIproj-3D or the 2D referenceimage PIproj-2D (step S16; refer to FIGS. 27 and 29). A partial regiondesignated by this ROI is enlarged, and superposed on the currentlydisplayed 3D reference image PIproj-3D or the 2D reference imagePIproj-2D, for example (step S17). Of course, such a re-projected imagemay be displayed differently from the panoramic image, displayedtogether with the panoramic image in a divided manner, or displayedusing a template consisting of a plurality of display blocks mappedalong the tooth row.

The image processor 56 then determines whether or not the foregoing setof processes should be ended, using information from the operator (stepS18). When being determined YES, the processing is returned to step S7,while being determined NO, the processing is returned to step S10 forrepeating the foregoing processing.

By the way, when it is determined at step S10 that the partial regionwill not be observed, the image processor 56 further determinesinteractively whether or not the currently displayed 3D automatic imagePIfocus is necessary to be rotated, moved, and/or enlarged or reducedfor display (step S19). If this determination is YES, the imageprocessor responds to a command to rotate, move, and/or enlarge orreduce the 3D autofocus image PIfocus, and displays such a processedimage (steps S20, S21). Then the processing proceeds to step S81 forrepeating the foregoing steps.

Of course, the display modes will not be limited to the foregoing, butcan adopt various other modes such as color representation.

When the operator commands to end the foregoing processing, the imageprocessor 56 ends the processing via performance at steps S18 and S7.

Incidentally, after setting at step S16, the processing may skip thedisplay at step S17 to directly proceed to step S19. In such a case, theROI which has been set may be displayed together with the rotated,moved, and/or enlarged or reduced image at step S21.

(Operation and Effects)

The panoramic imaging apparatus according to the preset embodimentprovides the following distinguished operations and effects.

First, differently from panoramic images produced by the conventionalpanoramic imaging apparatus, there can be provided, as a 3D autofocusimage PIfocus (a three-dimensional panoramic image), an image focusingon at least the entire region of the tooth row. According to this image,when the teeth are curved in the vertical direction, their realpositions and shapes are optimally focused every points (i.e., samplingpoints) in the vertical direction. In addition, the processing for thisoptimal focusing is automatically performed in response to only anoperator's intended command issued one time, so that the 3D autofocusimage PIfocus is displayed. That is, the autofocus function is provided.Additionally to this, the resultant 3D autofocus image PIfocus can berotated before being displayed and can be subjected to enlargement anddisplay of a ROI region set thereon, thus providing many variations forimage observation. Hence, it is easier for interpreters to veryprecisely examine the entire region of the tooth row, providing theexamination with higher accuracy. It is almost never required to re-trythe X-ray imaging, thus avoiding an amount of X-ray exposure fromincreasing, which is due to re-trying the imaging. The panoramic imagingapparatus according to the present embodiment is suitable for screening.

In addition to the above, changes in the enlargement factor, which arecaused due to changes of the rotation position, i.e., due to changes ofthe rotation center RC of the pair of the X-ray tube 31 and the detector32 during the scanning, are also corrected during the processing forproducing the 3D autofocus image PIfocus. Hence, image distortionresulting from changes in the enlargement factor is also removed orsuppressed, thus providing images which reflect therein real sizes andreal shapes of the teeth.

In obtaining panoramic images by the conventional panoramic imagingapparatus, changes in the enlargement factor result in the enlargementfactor changes between the molar teeth and the anterior teeth. Thiscauses a reduction in the degree of accuracy of measurement andunderstanding of distances and lengths on the image. In contrast, in thepresent embodiment, such problems are resolved, and images ormeasurement information with accuracy and very real sizes can beprovided, thus being fitted to detailed structural observation of thetooth row being imaged.

In particular, when the 3D autofocus image PIfocus is re-projected tothe 3D referential tomographic plane or the two-dimensional rectangularplane of a referential panoramic image as stated, the positionalcorrespondence to the three-dimensional autofocus image can be obtained,although a re-projected image involves a certain amount of distortion.Hence, for example, distances such as vertical lengths of teeth can bemeasured accurately.

Moreover, in the panoramic imaging apparatus of the present invention,the three-dimensional positions of the X-ray tube 31 and the detector32, which are moved during the data acquisition (scanning), arepreviously known. This means that it is not necessary to previouslymeasure information indicative of distances of tomographic planes(sections) in the imaging space by using a phantom, which is seen in theconventional case. From this point of view, operators' operability isimproved and calculation load of the image processor 56 is alleviated.

In consequence, there can be provided three-dimensional panoramic imagesin which the entire region of the tooth row is optimally focused, theactual states (positions and shapes) of the tooth row arethree-dimensionally depicted with higher precision, and distortion ofthe image which is due to differences in the enlargement factor isremoved as much as possible.

(Variations)

The foregoing embodiment has described the example that providesoptimally focused three-dimensional images of the tooth row of oralcavity of an object. This example can be developed into various othermodes. By way of example, there is the use of a marker (or X-raymarker). The marker, which is made of radiation-absorbing materialhaving an appropriate X-ray absorption factor, is arranged in the oralcavity. In this arranged state, like the foregoing embodiment, data areacquired to positionally recognize the marker and focusing and imaging asection including the marker.

FIG. 30(A) exemplifies a marker. As shown, a clip 70 has two squarechips 71 serving as markers and a wire member 72 with a spring mechanismconnects and holds the two chips 71 such that the chips are opposed tobe located alternately. The chips 71 are made of a material whose X-rayabsorption factor is higher than that of the oral cavity, whereby thechips can function as markers to the X-ray.

FIG. 30 shows, in its (B) and (C) sections, a state where the clip 70 isarranged at part of the tooth row of an object. The two chips 71 arefixedly arranged to be alternately on the front and rear sides of teeth(teeth row) such that the chips 71 differ in positions along the toothrow direction and pinch the gum with the help of the wire member 72. Inthis arrangement, the foregoing autofocus processing shown in FIG. 10provides a three-dimensional understanding of the two chips 71 andproduce two images, respectively, optimally focusing sections CR1 andCR2 which three dimensionally contain the respective chips 71 (refer toFIG. 30(C)). During this production, frequency characteristic patternsshown in FIG. 31 are adopted, in which two profiles PR4 each showingpeaks of the frequency characteristics of the chips 71 appear on bothsides of the frequency characteristic PR2 of the cancellous bone. Theimage processor 56 refers to the two profiles PR4 profile by profile toperform the reconstruction similarly to the foregoing embodiment. Hence,the three-dimensional positions P1real of the respective two chips 71(P1′real; refer to FIGS. 20-22) and two optimally focused images (orpartial images) of the sections CR1 and CR2 containing those positionscan be provided.

In producing the two optimally focused images, it is able to understandpositional information P from a distance D_(BN) between the positions ofthe two chips 71 in the back and forth direction of the tooth row. Andan amount of shift, L_(BN), of the chips 71 in the lateral direction(that is, a direction along the tooth row) is also known from the designof the clips. Hence, the image processor 56 can calculate the thicknessof the alveolar bone pinched by the two clips 71. In addition todiagnosing the alveolar bone, it is reliably select a sectional positionto be observed of the alveolar bone in the thickness direction thereof.

FIG. 32 shows another marker. This marker, which is used tightly coverthe face of a patient being examined, is a mesh-like and expandable mask80. This mask 80 is made of a wire material whose X-ray absorptionfactor differ from that of the oral cavity. Attaching the mask 80tightly to the face is equivalent to draw grid lines on the face withthe use of an X-ray maker. Simply these lines may be parallel with eachother. Another example of this marker attached on the face is shown inFIG. 33, where an X-ray absorption material, such as barium, is appliedto the face in a linear pattern or a grid pattern. A grid line 81 shownin FIG. 33 functions as a marker to the X-ray. Another marker applied tothe face may be quick-drying cosmetic liquid with granular X-rayabsorbent mixed therewith. This cosmetic liquid is applied to the face,so that the granular X-ray absorbent becomes a marker on the face.

The foregoing mesh-like marker 80, the marker 81 involving theapplication of the X-ray absorbent, and the granular marker provide aprofile PR6 in the frequency characteristics, as shown in FIG. 31. Theimage processor 56 uses this profile PR6 to perform the foregoingauto-focusing reconstruction. As a result, there can be provided notonly three-dimensional positions of the lines of the marker but also anoptimally focused image taken along a surface of the three-dimensionalpositions, that is, an X-ray transmission image of the surface of theobject's face. Displaying such an image on the foregoing 3D autofocusimage in a superposed manner, an image which can in substitution forcepharometric images can be provided.

By the way, in the foregoing dental panoramic imaging apparatus, thepair of the X-ray and the detector may be installed on the ceiling. Asan alternative, the apparatus can be mounted on an examination car orinstalled in a hose, if being compact in size and movable in structure.

The detector which can be employed in the radiation imaging apparatus ofthe present invention is not limited to the digital type of CdTedetector, but photon-counting type detectors can also be employed. Suchphoton-counting type detectors are known by for example PatentPublication JP-A-2004-325183.

The detector used by the radiation imaging apparatus according to thepresent invention is not always limited to use of the same type ofdetector. It is necessary to change an amount of energy of the X-raydepending on types of objects being imaged, so that material of theX-ray detecting elements may be selected in accordance with X-rayabsorbing coefficients tuned to the necessary amount of X-ray energy.When it is necessary to generate larger amounts of energy, materialssuch as LaBr3, CdTe, CZT, and GOS can be adopted for the X-ray detectingelements. By contrast, for smaller amounts of X-ray energy, materialssuch as Si, CdTe, CZT, and CsI can be adopted for the X-ray detectingelements.

Further, the display mode is not limited to displaying thethree-dimensional panoramic image (surface image). For example, from theprofile in the tomographic plane positions versus the values of sums ofsquares of amplitude shown in FIG. 17, a width assumed to be focused canbe obtained from the tomographic planes and the frequencycharacteristics, and using the width, the thicknesses of a tooth and/orthe alveolar bone, that is, the thicknesses of those in the depthdirection, can be measured. When a configuration to obtain thismeasurement information can be applied, together with the foregoingphoton-counting type detector, to the alveolar bone close to the firstfalse molar, bone mineral content can be measured quantitatively.

When the imaging according to the present invention is applied to themandibular antra or portions close thereto in the oral cavity, it ispossible to provide, to some extent, image information about thestereoscopic structure of the mandibular antra. Moreover, comparing abilateral difference in the image makes it possible to detectinflammation in the mandibular antra (empyema) in a more accurate mannermore than the conventional. Similarly, when the imaging according to thepresent invention is applied to the carotid artery or portions closethereto, calcification of the carotid artery, which is assumed to be oneof the reasons for arterial sclerosis, can be clearly displayedthree-dimensionally, providing more accurate diagnostic informationcompared to the conventional.

The radiation imaging apparatus according to the invention will not belimited to applications for the dental panoramic imaging apparatus, butcan be put into practice in applications where the tornosynthesistechnique is used to understand the three-dimensional shapes (positions)inside an object. Such applications include mammography using thetornosynthesis technique and an X-ray scanner for lung cancer in themedical field. Further, the radiation imaging apparatus according to theinvention can be applied to nuclear medicine diagnosis apparatusescalled emission CT (ECT) apparatuses such as a gamma camera and a SPECTapparatus. In this application, gamma-rays radiated from RI(radioisotope) administered into an object are acquired by a detectorvia a collimator with holes opened in a designated direction. In thiscase, the RI and the collimator compose the radiation emitting source.

In addition, the number of detectors mounted in the radiation imagingapparatus of the present invention is not always limited to one, but maybe two or more. Such two or more detectors can be driven as one unit orin parallel in a modality.

The radiation imaging apparatus of the present invention can also beapplied to industrial applications, which include acquiring informationabout contents of products or commodities carried by a belt conveyor andpositions of such products or commodities, inspecting three-dimensionalstructures of a flexible substrate connected to a flat panel display,acquiring information about three-dimensional distributions and sizes ofholes in a mold, and acquiring positional information of contents inbaggage screening in the airport. Objects being imaged can be movedlinearly, in a circle, in a curve or in other ways. That is, the 3Dreferential tomographic plane can be set as a tomographic plane orsection which is flat, cylindrical, or curved.

In particular, in the foregoing industrial applications, an object beingimaged is allowed to move relatively to the pair of the X-ray tube andthe detector, if necessary. Further, some reasons in the mechanicaldesign permit only the detector to be moved relatively to an objectbeing imaged or a patient and the radiation source.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide aradiation imaging technique which is able to optimally focus on theentire region of an object being imaged in a three-dimensional panoramicimage in a state where the actual state (position and shape) of theobject is three-dimensionally depicted with higher accuracy anddistortion due to changes in the enlargement factor is removed orreduced well, thus providing higher industrial availability.

DESCRIPTION OF REFERENCE NUMBERS

-   1 Dental panoramic imaging apparatus (radiation imaging apparatus)-   12 Control & calculation apparatus-   14 Imaging unit-   31 X-ray tube(radiation tube)-   32 Detector-   33 Collimator-   41 High-voltage generator-   53 Buffer memory-   54 Image memory-   55 Frame memory-   56 Image processor-   57 Controller-   58 Operation device-   60 Monitor-   61 ROM

What is claimed is:
 1. A data processing unit, wherein the dataprocessing unit processes electric digital frame data outputted from aradiation detector at a constant frame rate, wherein the radiationdetector being arranged to be opposed to a radiation source with anobject therebetween in an imaging space and the radiation sourcecomprises a point-like radiation portion emitting a radiation radiallytherefrom, the radiation detector receiving the radiation and outputtingthe frame data corresponding in intensity to the received radiation, theobject containing a curved portion, the curved portion being located ata curved tomographic reference plane imaginarily and three-dimensionallyset in the imaging space, a pair of the radiation source and theradiation detector being rotated around the object such that thetomographic reference plane is focused, the data processing unitcomprising: a first image producer producing an image of the tomographicreference plane based on the frame data; a searcher searching, based onthe image of the tomographic reference plane and the frame data, for afocused position in the curved portion, the optimal position beingpresent in each of radiated directions of the radiation emitted radiallyfrom the point-like radiation portion, each of the radiated directionspassing a corresponding one of a plurality of specified positions mappedvertically and horizontally in the image of the tomographic referenceplane; and a second image producer producing a focused image bycombining pixels of the focused positions in the curved portion, thefocused image reflecting therein an actual size and shape of the curvedportion in the imaging space.
 2. The unit of claim 1, wherein theradiation source is an X-ray tube emitting, as the radiation, X-rays;the radiation detector is a detector detecting the X-rays; the curvedportion of the object is a row of teeth in a mouse of a patient beingexamined; the tomographic reference plane is set along the row of teethand a three-dimensional tomographic reference pane having a curvedrectangular area; and the first image producer produces, as the image, apanoramic image along the tomographic reference plane based on the framedata and a tomographic image reconstruction method.
 3. The unit of claim2, wherein the radiation detector is a photon-counting type of X-raydetector which detects the X-rays and outputs the frame data.
 4. Theunit of claim 1, wherein the radiation source and the radiation detectorare opposed to each other such that an imaginary line passes theradiation source and the radiation detector, the imaginary line passingthrough a rotation center around which the radiation source and theradiation detector are rotated, and the rotation position ispositionally changed in the imaging space such that a distance betweenthe rotation center and the curved portion changes depending onadvancement in the rotation of the radiation source and the radiationdetector.
 5. The unit of claim 3, wherein the second image producercomprises removal means for removing noise from the focused image, thenoise being a pixel existing at an irregular position which has beensearched in the imaging space; and smoothing means for smoothing thefocused image by combining pixels which have been processed by theremoval means.
 6. The unit of claim 1, wherein the plurality ofspecified positions are spaced apart from each other to have apredetermined distance therebetween in the tomographic reference plane.7. The unit of claim 1, wherein the second image producer produces thefocused image in a whole area of which the actual size and shape of thecurved portion in the imaging space is reflected.